A freshly formed, silica-containing, glass surface will include a distribution of reactive sites that may include structural surface defects. It is well known that atmospheric moisture will react with these sites, leading to the attachment of hydroxyl groups to silicon atoms. As a result, a glass surface that has been exposed to the ambient atmosphere for a substantial period of time may be described as a "silanol-terminated" surface.
It is conventional to use silane coupling agents, such as RSi(OMe).sub.3 in the example below, to improve the adhesion of polymers to a silanol-terminated surface. Thus, for example, the water-mediated reactions: EQU RSi(OMe).sub.3 +H.sub.2 O+HOSi.ident..fwdarw.MeOH+[RSi(OMe).sub.2 ]OSi.ident.+H.sub.2 O EQU 2[RSi(OMe).sub.2 ]OSi.ident.+H.sub.2 O.fwdarw.2MeOH+.ident.SiO[R(OMe)]SiOSi[R(OMe)]OSi.ident.
provide a cross-linked siloxane surface that contains covalent coupling points R for a subsequent organic reaction with a polymer.
Such reactions may be useful for promoting the adhesion of an optical fiber to a polymeric jacket material. Even absent such jacket material, reactions of this kind may be useful for attaching chemical species to the glass surface that are effective for reducing the affinity of the surface, or of the glass-polymer interface, to water. For both of these reasons, a surface reaction of this kind may be able to impart desirable properties to glass fibers for use, e.g., in fiber-optic cables.
However, a surface treatment that relies upon the availability of a silanol-terminated surface will not, in general, be the most economical or effective treatment to use in optical fiber manufacture. This is because in large-scale fiber-making operations, the application of the polymeric coating is typically integrated in a continuous process that begins with drawing of fresh fiber from a heated preform, and ends with winding the coated fiber on a spool. The freshly drawn glass will not have a silanol-terminated surface. Instead, it will have a nearly dehydroxylated surface that is partially terminated by strained and unstrained siloxane rings.
In fact, there are known surface reactions that can attach organic species directly to a dehydroxylated glass surface. These reactions are described, for example, in L. H. DuBois et al., J. Amer. Chem. Soc. 115 (1993) 1190-1191, and L. H. DuBois et al., J. Phys. Chem. 97 (1993) 1665-1670. As discussed there, an alkoxy silane can react directly with a type of surface defect referred to as a "strained siloxane dimer ring," or, in reference to its geometrical configuration, as an "edge-shared tetrahedral defect." However, the resulting surface product contains a hydrolyzable alkoxy ester. This is undesirable because it may provide a nucleation site for the condensation of water. This, in turn, can weaken an optical fiber by way of a stress-induced corrosion mechanism. Moreover, the alkoxy ester is undesirable as a coupling point between the glass surface and a polymeric coating, because the adhesive bond to the polymer will be hydrolytically unstable. The surface reaction and subsequent hydrolysis are illustrated in FIG. 1.
Thus, practitioners in this field have hitherto failed to provide an effective chemical treatment for dehydroxylated glass surfaces, that affords protection from moisture and can promote adhesion to polymeric coatings.